This invention relates to a method and apparatus for removing volatile contaminants from the phreatic zone, also known as the zone of saturation, that is, ground water.
Contamination of the phreatic zone with potentially hazardous materials is a common problem facing industry, the government and the general public. Frequently, as a result of spills, leakage from storage facilities or surface discharges, contaminants percolate into ground water, thereby posing a health threat to drinking water supplies. While groundwater is not as susceptible to pollution as surface water, once polluted, its restoration is difficult and long term. Various methods for withdrawing and treating contaminated groundwater have met with limited success. Typically, groundwater is removed from the phreatic zone, treated and then returned to the phreatic zone. This method involves great expense.
One previous attempt at solving these problems involved a closed-loop device which included one or more contaminant withdrawal wells surrounded by multiple air injection wells connected by a conduit. This device utilized the principle of sparging. Sparging is the process of forcing air through water to remove undesirable volatile organic contaminants (VOC's). The above described device formed air channels within the phreatic zone that were distributed in three-dimensions. This approach may lead to uncontrolled distribution of contaminants.
The method and apparatus of the present invention utilizes the principles of controlled two-dimensional sparging. The method involves installing one or more air injection wells and pressure relief wells in the phreatic zone. The pressure relief wells are porous within the phreatic zone but impermeable above the phreatic zone. Air is forced into the air injection well through the phreatic zone towards the pressure relief wells. The arrangement creates an air and water mixture in the phreatic zone and the pressure relief wells, where the volatile contaminants are stripped from the phreatic water, removed and/or treated along with the injected air. As the contaminated phreatic water moves through the arrangement of air injection wells and pressure relief wells by natural flow the phreatic water is cleansed of volatile contaminants.
When operation of the present invention is initiated, the injected air spreads in all directions, in a three-dimensional manner, from the injection well. Additionally, the water pressure at the base of the pressure relief wells is significantly higher than it is at the top of the water table within the relief wells. However, once the injected air reaches the pressure relief wells and forms an air/water mixture within the pressure relief well, the density of the water column within the relief wells drops significantly. As the density of the water column within the pressure relief wells starts to decline, the pressure at the base of the pressure relief well decreases significantly. This causes the air in the phreatic zone to tend toward the relief wells much more readily than on initial startup, because the air seeks to move in the direction of lower pressure area offered by the pressure relief wells. Therefore, the injected air tends to travel more directly towards the pressure relief wells, rather than in all directions.
If the injection well and the pressure relief wells are ganged in a line, the resultant air channels would be formed in two directions (thus, in a two-dimensional single plane) from the injection well to each of the relief wells. Thereby, approximating a two-dimensional sparging region, rather than a three-dimensional region. This arrangement may be utilized especially well when the ground water moves perpendicularly across the air path.
The present invention obviates the need for a pump at each relief well to assist the contaminated gas in escaping from each pressure relief well for treatment or storage. The air/water mixture achieved in each relief well rises above the water table and forces the contaminated air to exit the relief well at the ground surface. The contaminated air may then be collected, and may be treated and/or returned into the injection well if desired.
Objects and advantages of the present invention will be readily apparent upon a reading of the following description.